An ink-jet printer is a well-known one of image-forming apparatuses (or image-recording apparatuses) such as printers, facsimile machines, and copiers. The ink-jet printer performs recording on a recording medium such as paper (recording paper) by ejecting ink from a recording head. The recording medium is not limited to paper. The recording medium refers to a medium on which an image is formable. The recording medium may be an OHP (overhead projector) sheet, for instance. The ink-jet printer can record a high-definition image at high speed with low running cost and reduced noise. Further, the ink-jet printer enjoys another advantage of easiness in recording a color image using multiple color inks.
A well-known ink-jet head includes, as a part for generating energy to eject ink, a piezoelectric actuator such as a piezoelectric element, a thermal actuator, a shape memory alloy actuator, or an electrostatic actuator. The thermal actuator utilizes the phase change of liquid caused by film boiling using an electrothermal transducer such as a heat element. The shape memory alloy actuator utilizes the phase change of metal caused by a change in temperature. The electrostatic actuator employs an electrostatic force.
There is an image-forming apparatus of an electrophotographic type. This type of image-forming apparatus forms a toner image on a photosensitive body using a charging unit, an exposure unit, and a developing unit provided around a photosensitive body drum, transfers the toner image to paper by a transfer unit, and fixes the toner image on the paper by a fixing unit.
Paper conveying units for such an image-forming apparatus are provided. Japanese Laid-Open Patent Application No. 7-53082 (Prior Art 1) discloses a paper conveying unit that conveys a sheet of paper fed at an angle from above directly in a horizontal direction and ejects the sheet of paper after recording. Meanwhile, Japanese Laid-Open Patent Application No. 8-295438 (Prior Art 2) discloses a paper conveying unit that changes the conveying direction of a sheet of paper fed substantially vertically from below by substantially 90 degrees using a guide member having a section with a round profile, and ejects the sheet of paper after recording.
Two types of conveyors may be used for conveying a sheet of paper. The first type of conveyor employs conveying rollers to convey a sheet of paper. Meanwhile, Prior Art 1 and Prior Art 2 disclose the second type of conveyor with an endless charged belt. The second type of conveyor causes a sheet of paper to adhere electrostatically to the charged surface of the charged belt, and rotates the charged belt with the sheet of paper adhering thereto so as to convey the sheet of paper. Thereby, this type of conveyor prevents the sheet of paper from being detached from the charged belt, thus maintaining a high flatness of the sheet of paper.
The image-forming apparatus of Prior Art 2 horizontally conveys the sheet of paper fed substantially vertically from below by changing the conveying direction of the sheet of paper by substantially 90 degrees by causing the fed sheet of paper to come into contact with a conveying guide formed along the surface of the conveyor belt (or a conveying path with a round profile).
In the case of horizontally conveying a sheet of paper fed substantially vertically in an upward direction by changing the conveying direction of the sheet of paper by substantially 90 degrees while guiding the sheet of paper by a conveying guide in a conveying path having a section with a round profile as in the image-forming apparatus of Prior Art 2, if the sheet of paper is plain paper, the resistance generated at the conveying guide is low when the sheet of paper comes into contact with the conveying guide. As a result, the sheet of paper can be conveyed with high accuracy without any problem even with a conveying belt with a low coefficient of friction μ.
On the other hand, thick paper such as glossy paper for improving image quality or a card is used particularly in an ink-jet recording apparatus. If such thick paper is used, the resistance generated at the time of changing the conveying direction of the sheet of paper becomes high so that the generated resistance exceeds conveying power if a conveyor belt with a low coefficient of friction μ is employed. This results in the phenomenon that the sheet of paper slips on the conveyor belt.
Thus, if a conveyor belt with a low coefficient of friction μ, which is prevented from generating large conveying power, is employed in conveying a sheet of paper that requires large conveying power, the conveyance of the sheet of paper becomes unstable so that a paper jam is more likely to occur.
According to ink-jet recording, ink adheres to paper. Therefore, when an image is formed on the paper, moisture included in the ink causes the paper to stretch. This phenomenon is referred to as cockling. Cockling causes paper waviness so that the distance between a nozzle of a recording head and a paper surface varies depending on a position on the paper surface. Cockling may worsen to such an extent that, in the worst case, the paper comes into contact with the nozzle surface of the recording head. As a result, not only the nozzle surface of the recording head but also the paper itself may be contaminated so that image quality is degraded. Further, ink droplet misdirection may be caused by the effect of cockling.
According to a conventional ink-jet recording apparatus, paper feeding is performed by rollers. A pair of rollers is provided on each side of an image printing region (where a recording head performs printing) in a paper feed direction. One of the pairs is the combination of a spur and a roller. According to this configuration, however, the accuracy of paper feeding can be guaranteed only when paper is held between each pair of rollers.
Due to a recent demand for an increase in the printing region, however, some ink-jet printing apparatuses perform printing with paper being held by only one of the two pairs of rollers, which is a state that cannot guarantee the accuracy of paper feeding, in order to secure the printing region. With paper being held by only one of the two pairs of rollers, however, the occurrence of paper flotation cannot be handled or the accuracy of paper feeding cannot be guaranteed because of insufficient conveying power. This results in the degradation of image quality.
Therefore, an ink-jet recording apparatus that includes an endless charged belt to maintain the flatness of paper has been proposed. Such an ink-jet recording apparatus causes paper to adhere electrostatically to the charged surface of the charged belt, and rotates the charged belt with the paper adhering thereto so as to convey the paper. Thereby, the ink-jet recording apparatus prevents the paper from being detached from the charged belt, thus maintaining a high flatness of the paper. Such an ink-jet recording apparatus is disclosed in Japanese Patent No. 2897960 (Prior Art 3) and Japanese Laid-Open Patent Application No. 7-53081 (Prior Art 4).
In the case of conveying paper electrostatically adhering to a conveyor belt as in the above-described conventional ink-jet recording apparatus, the flatness of the paper is directly linked to the flatness of the conveyor belt.
In this case, the conveyor belt engages at least two rollers to extend therebetween so that a chord part of the conveyor belt opposes an image printing region. However, the conveyor belt is prone to have wrinkles in its chord part, and when the conveyor belt is rotated, the conveyor belt may undulate in its chord part. As a result, the flatness of the conveyor belt may decrease.
Thus, even in the case of employing a conveyor belt, a decrease in the flatness of the conveyor belt causes a variation in the distance between a recording head and paper, thus degrading image quality.
Further, in the case of conveying paper electrostatically adhering to a conveyor belt as in the above-described conventional ink-jet recording apparatus, in order to stack the paper in a paper ejection part, the paper is separated from the conveyor belt by self stripping. Inelastic paper or paper that has lost elasticity with a solid image formed thereon, however, is prevented from being separated from the conveyor belt by self stripping, and remains adhered to the conveyor belt. As a result, a paper jam occurs.
Furthermore, in the case of conveying paper electrostatically adhering to a conveyor belt as in the above-described conventional ink-jet recording apparatus, it is necessary to secure an electrostatic adhesive force by the conveyor belt to cause the paper to adhere to the conveyor belt. In the conventional image-forming apparatus, however, the conveyor belt is a single-layer belt so that a sufficient electrostatic adhesive force to convey the paper stably cannot be obtained.